Summary Northwest Airlines Flight 11 (NWA11), a Boeing 747-400 aircraft, was en route from Detroit Metropolitan Wayne County Airport, Michigan, to New Tokyo International Airport, Japan, and was initially assigned flight level (FL) 310. While in Winnipeg Area Control Centre (ACC) airspace, NWA11 was cleared to FL350. This altitude was passed to the Edmonton ACC and to a La Biche controller, who annotated the flight progress strip with the new assigned altitude. Subsequently, the same controller, in error, changed the altitude on the strip back to FL310. A Syncrude Citation 560 (SYN21) was en route from Calgary International Airport, Alberta, to Fort McMurray Airport, Alberta, and was assigned FL350. Both aircraft were handed off to and accepted by a second and shortly thereafter a third La Biche controller at FL350. Both aircraft communicated with this controller, identifying themselves in the appropriate manner with their call signs and flight level. When NWA11 initially called the La Biche controller, the controller did not verify the strip and make the appropriate check next to the altitude in the altitude box. At about 1409 mountain standard time, both aircraft received and reacted to a traffic alert and collision-avoidance system resolution advisory. SYN21 descended, and NWA11 climbed. Separation was reduced to less than 2000 feet vertically and less than nautical mile horizontally. Ce rapport est galement disponible en franais. Other Factual Information The first La Biche sector controller (controller No 11) had medium traffic level and workload. He was working the sector by himself and did not have the data position staffed. About 30minutes before ending his shift, controller No 1 received eight estimates in less than 10minutes and two requests for altitude changes, including an Edmonton VOR (very high frequency omni-directional radio range) estimate on NWA11 at FL310. During this time, while he was attempting to establish a mental picture of the traffic flow, he was advised that NWA11 was cleared from FL310 to FL350. He annotated the flight progress strip (FPS) with the appropriate change of altitude. United Airlines Flight 881 (UAL881) had also requested FL350. This request required a rerouting and coordination with the Whitecourt sector. Since the Whitecourt sector did not have an FPS on UAL881, controller No 1 elected to coordinate face-to-face with the Whitecourt controller. He received approval for the route change, made the revision on the FPS, and handed the FPS for UAL881 to the Whitecourt controller. Controller No 1 then waited for new FPSs to be printed by the computer system. Shortly after returning to the La Biche sector position with the new FPSs, controller No 1 became uncertain as to which aircraft had been cleared at FL350. He concluded that he had written FL350 on the NWA11 FPS in error and crossed that figure out and wrote in the original altitude of FL310. Controller No 1 had also received an estimate from the Red Deer sector controller for SYN21. SYN21 was estimating a position 30nautical miles (nm) east of the Edmonton VOR at 1412 mountain standard time at FL350.2 This position was at the approximate point at which SYN21 would cross the flight path for NWA11. The NWA11 estimate was for the Edmonton VOR at 1412. Because controller No 1 had earlier re-entered FL310 in place of FL350 on the NWA11 FPS, he was unaware of a possible conflict and advised the Red Deer sector that the assigned altitude for SYN21 was acceptable. When relieved at 1400, controller No 1 briefed his relief (controller No 2) concerning the traffic situation and commented on SYN21 and a southbound Aeroflot aircraft at FL350, but advised that there was no conflict. No specific comment was made during the handover briefing regarding NWA11. Controller No 2 was on duty for about three minutes when, because of a supervisory decision, he was relieved by another controller (controller No 3). The handover briefing concentrated on the Aeroflot flight at FL350 and on SYN21 coming into the sector at FL350. Although the controllers did not consider this to be a conflict, they saw a need to monitor progress of the two aircraft until lateral separation was achieved. A short time later, controller No 3 accepted the handoff of NWA11 and then SYN21. Both aircraft reported to controller No 3 with their call signs and clearly stated their altitude of FL350. Controller No 3 did not read back the altitudes, nor did he make the required checkmark on the FPS beside the altitude box. At 1409 (seeFigure1), SYN21 advised controller No 3 that their traffic alert and collision-avoidance system (TCAS) showed converging traffic at the same altitude about eightnm away. Controller No 3 gave SYN21 an emergency descent to FL330 and directed NWA11 to turn right 90. Traffic at the time, and just before the loss of separation, was light with a light-to-moderate workload. The installation of a radar conflict-alerting system can provide a final ground-based line of defence. The TSB has made several recommendations regarding conflict-alerting systems to Transport Canada and Nav Canada. Implementation has been delayed several times and is not yet operational throughout Canada. Edmonton ACC management regularly conducts controller checks. However, FPSs are not routinely recovered and assessed to ensure that they are being properly annotated. Thus, if controllers are not following established procedures, no mechanism exists to identify and correct practices that do not conform to unit or Air Traffic Control Manual of Operations practices and standards.